Introduction

Basically, the problem asks how many combinations of two are there. In this case, we care about only modulo 60 of each value. Using a hash table to count modulo 60 values is a key to solve this problem.

Problem Description

You are given a list of songs where the i-th song has a duration of time[i] seconds.

Return the number of pairs of songs for which their total duration in seconds is divisible by 60. Formally, we want the number of indices i, j such that i < j with (time[i] + time[j]) % 60 == 0.

Constraints:

• 1 <= time.length <= 6 * 10**4
• 1 <= time[i] <= 500

https://leetcode.com/problems/pairs-of-songs-with-total-durations-divisible-by-60/

Examples

Example 1
Input: time = [30,20,150,100,40]
Output: 3
Explanation: Three pairs have a total duration divisible by 60:
(time[0] = 30, time[2] = 150): total duration 180
(time[1] = 20, time[3] = 100): total duration 120
(time[1] = 20, time[4] = 40): total duration 60

Example 2
Input: time = [60,60,60]
Output: 3
Explanation: All three pairs have a total duration of 120, which is divisible by 60.

Analysis

The first step is to count modulo 60 values. If modulo 60 is 0 or 30, within that value, two pairs can be created. So, n * (n - 1) / 2 is the number of combinations. If modulo 60 and 60 - modulo 60 are there, for example, 20 and 40, combinations are number of 20 times number of 40. To avoid the pair doubly, up to 30 condition is added.

Solution

class PairOfSongsWithTotalDurationsDivisibleBy60:
    def numPairsDivisibleBy60(self, time: List[int]) -> int:
        counter = collections.Counter([t % 60 for t in time])
        result = 0
        for k, v in counter.items():
            if k == 0 or k == 30:
                result += (v * (v - 1) // 2)
            elif k < 30 and 60 - k in counter:
                result += (v * counter[60 - k])
        return result

Complexities

• Time: O(m + n) – m: length of given array, n: distinct values of x % 60
• Space: O(n)